The present invention relates generally to the field of cardiology and, more particularly, to a method and system of processing an electrocardiogram signal to detect T-wave alternans by aligning alternating beats to a cubic spline. More accurate detection and quantification of alternans within the ST-segment and T-wave of the signal is then possible upon the aligned beats.
In the field of electrocardiography, electrical alternans are the differences in electrical potential at corresponding points between alternate heartbeats. T-wave alternans or alternation is a regular beat-to-beat variation of the ST-segment or T-wave of an ECG which repeats itself every two beats and has been linked to underlying cardiac instability. A patient""s odd and even heartbeats may therefore exhibit different electrical properties of diagnostic significance which can be detected by an electrocardiogram (ECG).
The presence of these electrical alternans is significant because patients at increased risk for ventricular arrythmias commonly exhibit alternans in the ST-segment and the T-wave of their ECG. Clinicians may therefore use these electrical alternans as a noninvasive marker of vulnerability to ventricular tacharrhythmias. The term T-wave alternans (TWA) is used to broadly denote these electrical alternans. It should be understood that the term encompasses both the alternans of the T-wave segment and the ST-segment of an ECG.
It may however be both difficult to detect TWA and difficult to quantify the magnitude of TWA since the magnitude of the phenomena is typically less than one hundred microvolts. Differences of this magnitude between ECG signals are difficult to differentiate from baseline wander, white noise, or from other artifacts such as patient movement or other irregularities in the heartbeat.
The current method of detecting TWA involves receiving an ECG signal and, from this data, calculating both an odd and an even median complex using the respective incoming odd and even signal data. The odd median complex is then compared with the even median complex to obtain an estimate of the amplitude of beat-to-beat alternation in the ECG data. The maximum alternation amplitude observed between the end of the QRS-complex and the end of the T wave is defined as the T-wave alternans value. A TWA is present if this value is greater than some threshold value determined by a clinician.
In the prior art the baseline wander was removed by calculating a cubic spline based on points measured between the P-wave and the QRS-complex of three consecutive QRS complexes. The values generated by this spline curve were then subtracted from the corresponding values of the incoming beat data. Since points in the isoelectric area preceding the QRS complex are used to calculate the cubic spline, this method does not properly correct for baseline wander between the end of the QRS-complex and the end of the T-wave.
To better correct baseline wander it would be preferable to use an additional point after the T-wave in calculating the cubic spline correction. However the amplitudes of the isoelectric areas before the QRS-complex and between the T and P-waves differ. The isoelectric area before the QRS-complex is influenced by the atrial repolarisation. Other reasons for different amplitudes in both xe2x80x9cisoelectric areasxe2x80x9d could be a short PR-interval or a merging of P- and T-waves. Applying the cubic spline correction algorithm to points before the QRS-complex and also to points after the T-wave will cause the algorithm to produce artificial baseline wander, and therefore to produce incorrect T-wave alternans values. As a result, a more effective means of aligning odd and even heartbeats is needed in order to obtain more accurate TWA values.
The invention offers a technique for detecting T-wave alternans by aligning alternating heartbeat data, i.e. odd and even beats. In a preferred embodiment of the invention, a digitized ECG signal is received for processing. The ECG data is used to calculate an odd and even median beat and a target cubic spline which is then used to align an odd and an even median beat complex. The odd median complex is then compared with the even median complex to obtain an estimate of the amplitude of beat-to-beat alternation in the ECG signal.
The step of calculating a median complex may proceed as follows. A first array (representing the odd median complex) is initialized with the median of a plurality of odd complex values. A second array (representing the even median complex) is initialized with the median of a plurality of even complex values. The samples of a new odd beat of the ECG data are compared to corresponding values in the first array and, based on the comparison, the values of the first array are adjusted as follows. If a sample of the odd beat exceeds the corresponding value of the first array by a fixed amount, then the corresponding value is incremented by the fixed amount. In the other case the corresponding value is incremented by {fraction (1/32)}th of the difference between the sample of the odd beat and the corresponding value of the first array This process is repeated for other odd beats desired to be included in the calculation. This same process is then followed for the second array using the even beats.
Once the odd and even median complexes have been calculated they are then aligned. This alignment is accomplished by calculating a target cubic spline, an odd median complex cubic spline, and an even median complex cubic spline. The differences between the target cubic spline and both the odd median complex cubic spline and the even median complex cubic spline are then calculated. These differences are then subtracted, respectively, from the odd and even median beat data, to correct (align) them.
The effect of this alignment step is to minimize any residual baseline wander between the odd and even beat data. More accurate comparisons of the odd and even beat data may then be made.
In accordance with one aspect of the present technique, there is provided a method of calculating a reference function (in the preferred embodiment a target cubic spline) derived from odd and even beat data and useful for aligning odd and even median beat complexes.
In accordance with another aspect of the present technique, there is provided a method of processing ECG signals for alternating heartbeats and of aligning these alternating heartbeats using cubic splines. The method may be extended to incorporate the detection and quantification of differences, such as alternans, between the alternating ECG signals.
In accordance with another aspect of the present technique, there is provided a system for processing ECG signals for alternating heartbeats whereby the ECG signals are analyzed by processing circuitry to derive a reference function, the alternating ECG signals are aligned by the processing circuitry, and the aligned ECG signals are saved by memory circuitry or displayed by display circuitry. In addition, the system may be expanded to include analysis circuitry capable of processing the aligned ECG signals to determine the presence and magnitude of variations between the alternating signals such as T-wave alternans.
These and other features and advantages of the invention are described in detail below with reference to the figures in which like numbers indicate like elements.